Scope includes in std.functional

Trivial import scoping for `std.functional`. `functional` contains only "little helper functions" that are generally useful, so it is relativelly important for it to not systematically pull in the rest of the world.:

1. Move everything that's imported explicitly for `NaryFun` inside `NaryFun`.
2. Also scope `std.typecons` and `std.conv` while we are at it.

That's it.